1. Field of the Invention
This invention relates generally to feed shoes for feeding and depositing finely divided or particulate material into a die cavity for compacting. More particularly, the invention relates to a feed shoe that controls the delivery of particulate materials into the die cavity. This controlled delivery provides the particulate materials in the die cavity with a uniform density that is greater than what would otherwise be the bulk density of the particulate material or with a density gradient. The present invention also pertains to a process for precompacting particles in a die cavity to produce articles of uniform density or gradient density from particulate material.
2. Description of Related Art
In powder metallurgy and other technologies using particulate materials such as ceramics and carbides, products and parts are formed by pressing finely ground or atomized powders into a desired shape within a die cavity. Generally, the powders are compacted in the die cavity at room temperature and the then semi-dense compact is removed from the die and heated to bond the powders into a unified, dense mass. In powder metallurgy, the heat bonding procedure is generally known as sintering or in the case of ceramics and carbides, firing.
When these and similar procedures are employed, means are required for delivering measured amounts of powder or particulate to a die cavity on a powder press. Typically, feed shoes operate to deliver the powder or particulate material to the die cavity during the press cycle by using a gravity fill system. This system involves the movement of the feed shoe containing particulate material on a shuttle which slides the shoe forward along the table of the die press to a position at which the bottom feed hole of the feed shoe is exposed, overlies and registers with the die cavity furnishing enough loose powder to gravity fill the die cavity. Thereafter, the shuttle slides the shoe back along the table of the die press into a retracted position thereby cutting off the gravity flow of particulate material from the bottom hole of the feed shoe. The particulate material is then pressed into an article and the article is ejected from the die. The shuttle then slides the shoe forward along the table of the die press displacing the ejected article and again exposing the bottom hole of the feed shoe as it overlies and registers with the die cavity thereby allowing gravity to fill the die cavity once again. The feed shoe is retracted once again thereby cutting off the gravity flow of particulate.
These known simple gravity feed methods deliver particles into die cavities at a density that is roughly equivalent to the bulk density (density of a bulk volume of the particles) of the particulate material. Because of gravity, all areas of the die cavity do not fill uniformly. Thus they do not provide for a uniform density of particulate material within the cavity. Articles produced by these feed methods are many times unsatisfactory because when pressed, they produce compacted parts that are also non-uniform in density. These non-uniformly dense articles are then prone to stress related cracking, especially upon ejection from the die cavity. These cracks are often only visible after sintering. Typically, with an almost spherical powder such as for example, copper coated aluminum powder, the fill ratio of amount of powder in die to size of compacted part is 3:1 using gravity to fill the die cavity. That is, the three dimensional size of the powder in the die is three times that of the final compacted article.
Shaking or vibrating a feed shoe is frequently employed in the art for inducing a more regular flow of the particulate material into the die cavity. This, however, is time consuming and is generally not adequate to achieve uniform density of the particles in the die cavity or compacted articles of uniform density which are reproducible from article to article in a controlled manufacturing process.
Additionally, this shaking or vibrating of fine particles in known particle feed methods dislodges "fines" and dust from the particles which are then air borne to coat and contaminate the surrounding environs. Many of the particulate materials are frequently quite costly and in some cases toxic. Thus, the dust problem can represent a considerable economic loss or health hazard. Consequently, relatively elaborate and costly dust recovery systems and personnel safety precautions, such as filtered masks are presently used.
U.S. Pat. No. 4,813,818 to Sanzone discloses a feed shoe having a hopper for receiving powder materials from a source that communicates through a feeder tube with an enclosed filling chamber. The filling chamber is equipped with a vacuum. The vacuum is applied to assist the gravity flow of the powders through the feeder tube into the filling chamber. However, like the shaking or vibration techniques described above, such evacuation of the chamber does not provide for the uniform density of particulate material within a die cavity necessary to produce homogenous articles and materials that must exhibit uniform mechanical and physical properties. Examples of such articles or materials are those used in thermal management, etc. in which strict uniformity of properties (i.e., coefficient of thermal expansion, thermal conductivity, etc.) throughout the article or from article to article is required. Further, this chamber evacuation cannot be used to control the density of the particulate material in the die cavity and thereby create a density gradient of the particulate material and in the resultant compacted article.
Additionally, technologies exist for controlling and moving mechanical parts of die presses at much faster rates than those presently being employed. However, the rate at which die presses can produce articles is limited by the rate at which the die cavity can be filled with particulate materials. This rate is relatively slow using known feed shoes which use gravity to feed powders into the die cavity and does not allow the die presses to reach their maximum capacity for producing compacted parts and is even slower using vibratory methods.
Moreover, in the aforementioned and other known feed shoes, the step of retracting the feed shoe by dragging the feed shoe over the upper surface of the wear plate of the die table has been necessary to cut off the flow of particulate materials from the feed shoe. This retraction of the feed shoe after filling the die cavity, however results in the buildup of powders in the die near the trailing edge of the feed shoe. This friction induced "wedging" effect further exacerbates the problem of producing articles of non-uniform density upon compacting the particulate material in the die cavity, with all of the accompanying aforementioned disadvantages.